The goal of the project is to develop a new type of sensor for magnetic fields of biological origin. Biomagnetic fields can provide valuable and non-invasively diagnostic information. For example, magnetoencephalography (MEG) has been used for localization of the epileptic focus for surgical treatments of epilepsy. MEG is also one of the primary tools in studies of visual and auditory evoked fields, and cognitive processes, such as speech recognition and memory formation. Magnetocardiography (MCG) has been used for localization of the arrhythmic tissue in treatments of venticular tachucardia (VT), Wolff-Parkinson-White (WPW) syndrome, and other cardiac conditions. Typically, multi- channel systems with 30-100 discrete SQUID detectors are used for localization of the magnetic field sources. In many cases signal averaging is required to obtain an adequate signal to noise ratio. Cryogenic requirements result in high cost of these systems. The localization accuracy of the magnetic techniques is typically comparable to the accuracy of the corresponding electric techniques (EEG and ECG), which are substantially less expensive. This, so far, limited wide clinical use of biomagnetic diagnostic techniques. The proposed magnetometer will address the limitations of the existing technology. The sensitivity of the magnetometer will be higher than the sensitivity of a typical SQUID detector by two orders of magnitude. It will also allow continues mapping of the magnetic fields with spatial resolution of approximately 1mm3 over a volume on the order of 100 cm3. This will dramatically improve the localization accuracy of the magnetic field sources. It will be possible to configure the magnetometer as a variable order gradiometer using software techniques. It is also expected that the cost of the proposed magnetometer can be lower than the cost of the cryogenic SQUID systems. The sensor will be based on an optically pumped alkali metal spin magnetometer, which has a long history of use in physics. Improvement in the sensitivity will be achieved by a new design, based on recent theoretical and experimental developments in the field. The magnetometer will use K metal and He buffer gas. The buffer gas will slow the diffusion of K atoms without causing significant spin relaxation. The detection technique will be limited only by the spin relaxation rate, instead of the much larger spin exchange rate. Optical pumping and detection will be done with inexpensive diode lasers, which recently became available. Magnetic field readout will be performed using a linear detector array or a CCD camera. In this proposal period we plan to demonstrate the sensitivity and imaging capabilities of the detector using non-biological sources of the magnetic field.